Living since almost 15 years near Paris, in a few different places, regularly taking line B, C, D, M6, M7, M13, M1, some buses, as part of my regular commute, and have never encountered the shit you are talking about. I do know that some places are more problematic than others, by example M6 drivers regularly warn of pickpockets on saturdays (it's a touristic line, the only one I regularly take that have such announcements), sure not everything is perfect, but you are either biased (where did you live when you were in Paris?) or actually purposely actively trying to spread fud to insist like you are.